Controlled Synthesis And Catalytic Performance Of Hydrothermal Carbon Spheres Supported Cobalt Catalysts For Fisher-tropsch Synthesis

Nowadays,an increasing energy shortage has won attention for the development of new energy sources and the efficient use of fossil energy.Fischer-Tropsch synthesis?FTS?is an important catalytic process to produce clean fuels and high-valued chemicals from syngas.It means that in future FTS can be an important mean to replace fossil fuels to maintain energy supply.Co-based catalysts has been widely used in industry for excellent performance and cost-effective price.For Co-based FTS catalysts,catalytic activity and selectivity are strongly correlated with the electronic and geometric structure of cobalt nanoparticles,which depend on the particle size,morphology and crystallographic phase of the cobalt nanoparticles.Thermal decomposition,a mature method for obtaining high-quality nanoparticles,can precisely control the particle size,morphology and crystallographic phase of nanoparticles.In this paper,we applied thermal decomposition method into design and synthesis of FTS catalysts.A series of CoO/C catalysts were prepared using hydrothermal carbon spheres as support.The particle size,morphology and crystallographic of CoO nanoparticles were controlled by the nucleation and growth of particles.Using the obtained catalysts as model catalysts to in-depth study particle size,crystallographic and morphology effects in Co-based FTS catalysts.The main contents of this thesis are as follows:?1?Comparing the catalyst prepared by thermal decomposition method with the catalysts prepared by conventional incipient wetness impregnation and ultrasonic impregnation.It was found that the catalyst,prepared by thermal decomposition method,showed higher dispersion,resulting in higher CO conversion and C₅₊selectivity,lower CH₄ selectivity.?2?Cobalt catalysts with different cobalt particle sizes were prepared by thermal decomposition method using hydrothermal carbon sphere as support,Co?acac?₂ as cobalt source,benzylamine and o-dichlorobenzene as thermal solvents.It was found that cobalt source concentration and thermal solvents molar ratio could control the particle size of CoO,further to control the particle size of the metal Co particles.FTS catalytic performance results showed that the particle size of CoO increased with the increasing of cobalt source concentration and the decreasing of the benzylamine/o-dichlorobenzene molar ratio.The catalyst with Co particle size of 7.7nm showed the highest CTY value.Turnover Frequency?TOF?and C₅₊selectivity increased with Co particle size below 7.7 nm,and approached a constant value for even larger Co particles.CH₄ selectivity increased with decreasing Co particle size below 7.7 nm,and approached a constant value for even larger Co particles.?3?The morphology of CoO particles supported on CS was controlled by cobalt source type,thermal solvents ratio and heating method.The morphology of Co particles could be maintained by H₂/Ar reduction,further to study the crystal plane effect of Co-based FTS catalysts in the actual reaction conditions.The FTS catalytic performance showed that hcp-Co pyramid catalyst exposed hcp?101?crystal plane had the lowest apparent activation energy and the highest intrinsic activity;while the fcc-Co octahedral catalyst exposed fcc?111?crystal plane showed higher apparent activation energy and lower intrinsic activity.In situ diffuse reflectance Fourier transform infrared spectroscopy?in situ DRIFTS?also indicated that hcp-Co pyramid catalyst tended to bridge bonded CO,and fcc-Co octahedral catalyst tended to linear bonded CO.The Co particles of hcp-Co rodlike catalyst showed morphology evolution during reaction.A part of high active?101?crystal plane disappeared or transformed to lower active crystal plane,while the rodlike particles themselves had larger particle sizes,which resulted in intrinsic activity lower than that of fcc-Co octahedral catalyst at high temperature condition.